On-line measurements of the amount of dissolved solids of paper mill process waters, such as whitewater, graywater, and effluents, can provide the necessary feedback for optimizing retention, flocculation, and water flow in the paper mill. At present, on-line measurements do not provide the detail necessary for optimal control. This is particularly the case when measuring the total amount of dissolved solids in a liquid sample.
The importance of the management of the composition of industrial water streams is described by Simons, NLK Consultants, and Sandwell Inc., in a 1994 publication "Water Use Reduction in the Pulp and Paper Industry", Canadian Pulp and Paper Association, Montreal. The excessive build-up of dissolved solids in a process water stream may decrease process efficiency and increase corrosion, foaming, odour, pitch, precipitation, and scaling. A counter-current flow of water to pulp streams is a commonly used method to efficiently use water in pulp processing and papermaking to optimize the removal of dissolved solids. In order to prevent production problems related to the build-up of dissolved solids in process water it is necessary to efficiently remove and minimize the variation of dissolved solids in liquid samples. Garver et al. in a Journal entitled Tappi Vol. 80 Number8, pages 163-173, 1997 teach that the temporal or spatial variation in the amount of dissolved solids in a water stream may lead to manufacturing problems including precipitation, deposition, scaling and pitch formation.
One standard method for the examination of water and wastewater employed by the American Health Association measures the total amount of dissolved solids directly by gravimetric analysis after evaporation of a known volume of liquid after filtration.
The empirical estimation of dissolved solids using a conductivity measurement is an established technique employing a calibration between the dissolved solids and a conductivity measurement. This method is widely used as a relative measure of dissolved inorganic salts and many conductivity/TDS (Total Dissolved Solids) meters are available on the market. The relationship between dissolved solids and conductivity differs for each type of ion depending on the charge and size of the ion. Empirical constants to convert conductivity (mS cm.sup.-1) to dissolved solids (mg L.sup.-1) may vary considerably, i.e. between 0.55 and 0.9 depending on ion type, concentration and temperature, American Public Health Association, Standard Methods for the Examination of Water and Wastewater, American Public Health Association, American Water Works Association, Water Pollution Control Federation, Washington D.C. 1992, pp. 2-47. However, the amount of dissolved solids measured by conductivity is only reliable when specific inorganic salts dominate the dissolved solids present in the water. Conversely, conductivity measurements present a poor measure of the amount of dissolved solids when substances with little or no ionic charge contribute substantially to the amount of dissolved solids.
The principle disadvantages of using conductivity as a measure of the amount of dissolved solids are related to inaccuracies arising from the differences in the specific conductivity of different ions, association or chelation of positive or negative ions resulting in inactive ions, and the poor detection of organic acids and organic neutral substances. In a paper mill situation the relative ratio of dissolved inorganic salts to dissolved organic material varies dramatically depending on the location in the pulp processing sequence. For example, in a lignin retaining pulp brightening process, such as sodium hydrosulfite bleaching, the variation in the amount of dissolved solids may be largely related to the amount of bleach applied and the residual sulfur species resulting from hydrosulfite decomposition.
The patent literature describes applications using conductivity measurements to control water introduction, counter-current flow or sewer flow in pulp or paper processing. The objective of the control of the amount of dissolved solids using conductivity measurements has been to improve the washing, separation and removal of solids and to minimize scaling and deposition. Rosenberger (U.S. Pat. No. 4,096,028) discloses feed-forward control of the amount of dissolved material in a counter-current flowing liquid using conductivity measurements and flow rates. Sexton (U.S. Pat. No. 4,046,621) disclosed a feed backwards method for the control of pulp treatment using conductivity measurements. Heoksema et al. disclose an apparatus for conductivity measurements of pulp washing liquors from a drum type washer. Lisnyansky and Blaecha taught a control strategy for optimizing the efficiency of counter-current flow pulp washing based on a dilution factor or soda wash.
In a counter-current flow pulp treatment or washing not only the removal of dissolved ions may be controlled by a conductivity measurement but the accumulation of the water may also be measured and controlled. The benefits of maintaining a low or constant amount of dissolved solids are related to solubility equilibria which influence the extraction of unwanted material from pulp and also govern the deposition and precipitation reactions leading to unwanted scale and deposits.
The absorbance from selected wavelengths of the UV may be used as a measure of the relative quantity of extractives and lignin or carbohydrate derived components. Marcoccia et al. (U.S. Pat. No. 5,547,012) teach a method of control of kraft pulping by controlling the amount of dissolved organic material in a continuous digestor.
Sloan (U.S. Pat. No. 4,886,576) teaches a method for using the UV absorbance of lignin dissolved during digester cooking for control of pulp cooking parameters and refiner energy. Manook et al. (U.S. Pat. No. 5,420,432 or Cdn. U.S. Pat. No. 2,106,472) disclose an organic pollutant monitor based on UV absorbance measurements for the determination of the amount of organic matter.
Papermaker's demands for high speed and efficiency, flexible manufacturing, stringent quality standards, and environmental compatibility coupled with new developments in on-line process control are driving the development of new sensor technology for the paper machine wet-end. The need for better means for providing wet-end chemistry control is emphasized by recent reports that only 10% of the world's 150 newsprint paper machines operate at above 88% efficiency and over 60% operate under in the low efficiency range of below 82.5%. (Mardon, J., Chinn, G. P., O'Blenes, G., Robertson, G., Tkacz, A. Pulp and Paper Canada, 99(5) 43-46. (1998).
Nazair and Jones teach that wet-end variability arising from practical determinants and disturbances leads to variations in molecular and colloidal interactions that result in practical consequences in terms of the process and the product (Nazair, B. A; Jones, J. C. (Paper Technology 32(10) 37-41. 1991. Optimizing wet-end chemistry--the practicalities.). Practical determinants include the type of furnish, fillers, chemical being used, addition rates, addition points, refining, pH, temperature and consistency. Disturbances include broke, machine breaks, quality of materials, machine wear and seasonality. These variations may deleteriously effect system cleanliness, runability, first pass retention and product quality factors including formation, sizing, uniformity, strength, porosity and defects. The high capital cost of paper machines demands maximization of paper machine efficiency and quality. The papermaker will attempt to minimize system-input variation and counteract variation in practical determinants and disturbances so as to minimize variation and degradation of process efficiency and product quality.
The consequences of poor control of the variation, total level and composition of dissolved substances have been recognized by numerous authors. Gill teaches the importance of variation control of dissolved and colloidal substances in the paper machine wet-end. "Dissolved and colloidal substances (DCS) are released from the water phase from contaminated pulps or broke, and form deposits at the wet-end, press section, machine fabrics and rolls. These deposits cause: downtime; defective products; sheet breaks; frequent fabrics change." William E. Scott address problems related to wet-end chemistry control. Principles of Wet End Chemistry. Tappi Press, Atlanta, 1996. p 3. "Deposits and scale usually arise from out-of-control wet end chemistry. Typical examples include chemical additive overdosing, charge imbalances, chemical incompatibility and the shifting of chemical equilibria. All of these phenomena can lead to the formation of precipitates or colloidal aggregates that produce deposits and scale. While there are numerous approaches to treating the symptoms of deposits the best approach is to determine what is out of control and fix it."
One simple measure of the variability of the wet-end system chemistry is the level of dissolved organic and inorganic solids in the paper machine white water system. Tools that have become available for wet-end chemistry monitoring include retention monitoring, turbidity and electrokinetic potential (streaming current, cation charge demand, and zeta-potential) instruments. On-line instrumentation for monitoring and controlling the inorganic and organic dissolved and colloidal solids in a paper mill is at present limited to conductivity measurement or on-line charge measurement. While off-line total dissolved solids, turbidity, pitch counts, COD and TOC measurements may be used. In summary, the presently available means for on-line monitoring of wet-end chemistry fall short of providing reliable measurement of dissolved organic and inorganic solids.
Chemicals can provide control of the levels of DCS and deposit formation can be eliminated or reduced to tolerable levels by careful control of water flow and addition of chemicals for either dispersing or adsorbing and coagulating dissolved and colloidal substances. (Gill, R. S. Paper Technology, 37, July/August, 1996. 23-31. Chemical control of deposits-scopes and limitations.)
It is an object of the present invention to provide a method and an apparatus for on-line measurement of the amount of dissolved solids in a liquid sample, such as in a pulp or paper mill process water or effluent.
It is another object of the invention to provide an analyzer for total dissolved solids by combining conductivity and UV measurements of a liquid sample. In combination, these measurements are used to determine the total dissolved solids in a liquid using a mathematical relationship for expressing the relationship between variables. Furthermore, additional mathematical relationships are provided for estimating the relative contribution of inorganic and organic dissolved components, or ionic and non-ionic components.
According to a specific object of the invention an on-line measurement and control system for dissolved substances in paper mill process waters is provided. Environmental concerns and demanding manufacturing processes afford the development of sensors. In accordance with the invention the amount of dissolved solids is measured as a function of both UV absorbance and conductivity of the sample. High levels of dissolved solids and variation in the amount of dissolved solids leads to runability problems of paper machines. Thus, to improve the manufacturing process in a pulp and paper mill better control of the amount of dissolved solids in process water, such as white water, is needed.